The present invention relates in general to circuits for improving the quality of a television picture and, in particular to, a new and useful circuit for improving picture quality in a television receiver where light picture areas correspond to a high video signal level bounded by inclined rising and falling edges and where a pulse is added to the falling edge of the high video signal level.
As is known, the video signal obtained from the IF-stage is transmitted in a television receiver directly or over filters to the chroma-stages or the electron beam tube. The video signal originally supplied by a television station has a band width of about 5 MHz. Due to errors in the receiving station and band limitations in the transmission of the signal or filter with limited pass range, it is not possible in practice to reproduce the entire originally emitted frequency spectrum on the picture screen. The result is that steep black and white transitions are blurred or fine structures can no longer be recognized on the screen.
Various methods have been suggested in the literature to increase the picture quality on the picture screen by which the picture definition can be more or less improved. In the so-called aperture compensation method, the intensity of the electron beam is first reduced in those parts of the picture where the brightness passes from a low to a high level or vice versa by adding a twice differentiated video signal to the video signal, and then increasing it again or vice versa. Peak levels in the brightness of the compensated video signal may lead to beam currents which are substantially increased, compared to the beam currents existing in the original video signal, so that the picture elements on the picture screen increase again and the method does not lead to the desired result.
In order to eliminate this drawback, DOS No. 23 47 573 suggests a method for producing rapid brightness variations on a picture screen, where a control signal characterizing the amplitude value of the video signal is derived from the video signal, which is used to vary the horizontal deflection velocity. In this method the video signal is differentiated twice, and the deflection velocity of the electron beam is modulated with this signal in the line direction by a special deflection coil. In order to effect the modulation of the deflection velocity at the same point where the video signal jumps, it is necessary to delay the video signal by a delay line for a certain time interval. Since the video signal is fed in this method uninfluenced to the cathode, the deflection velocity modulation method does not lead to a variation of the size of the luminous spot, so that this method offers certain advantages over the aperture compensation method. A disadvantage of this method is that light or white areas of the reproduced picture are narrowed in the line direction, so that the information is falsified.
DOS No. 27 53 196 describes a method which uses the deflection velocity modulation method in combination with a waveform circuit. It starts from the known velocity modulation method, but it avoids the disadvantage that the white portions are narrowed in the video signal. The waveform circuit used contains an OR-gate circuit which receives the uninfluenced video signal and a video signal delayed over a delay line, so that a widened video signal appears at the output of the waveform circuit. In addition, the deflection velocity modulation method is used, so that a picture appears on the picture screen whose white portions are reproduced in the correct width, but the picture has still a greater definition. This method required a delay arrangement in the waveform circuit.